GB2092024A - Drawable protective coatings for container manufacture - Google Patents

Abstract

To protect foodstuffs against contamination in a drawn and ironed two piece can, the inside surface thereof has a protective resin coating (11) on the base metal (13). The coating is applied to a flat metal blank from which the container body is drawn and is baked before drawing at modestly elevated temperatures between approximately 150 and 315 DEG C, for evaporating solvents therefrom and for curing the resin components. The coating composition includes a phenolic resin, an epoxy resin, a polyvinyl chloride/acetate copolymer and a polyvinyl chloride dispersion resin dispersed in a solvent system; solvent impurities which can react with low molecular weight sulfides are excluded. The polyvinyl chloride dispersion resin included in the composition has an intrinsic viscosity of about 1.4.

Description

1

GB2 092 024A 1

SPECIFICATION

Drawable protective coatings for container manufacture

5 The present invention relates to drawable protective coatings for container manufacture. The 5

coating formulations disclosed herein are intended primarily for use in contact with foodstuffs and beverages.

Foods such as vegetables, fruits and soups have been packed for many years in steel or tinplate containers made by the so-called three-piece metal can technology. Such containers are 10 fashioned by forming a body cylinder from flat metal stock and joining the adjoining longitudinal 10 edges by soldering or welding. The body cylinder is then flanged and a bottom end affixed thereto by the process known as double seaming. The remaining end is affixed by the packer after the container is filled.

Three-piece metal containers can have protective enamels coated on them to prevent 15 corrosion, particularly when the containers are made from tin free steel (TFS). Cans are also 1 5 made from tin coated steel and these may or may not have protective enamels applied to them, depending on the type of food product that is to be packed.

The protective enamels used in metal cans are carefully selected to provide the required corrosion resistance and in addition to avoid impairing the flavor of the food product. Enamels 20 that are typically used include phenolic resins, epoxide resins, amine type resins, oleoresinous 20 materials such as China wood oil and vinyl type resins such as polyvinyl chloride or vinyl chloride/vinyl acetate co-polymers. These are formulated in specific solvent mixtures (based for example on ketones, esters, alcohols and aromatic or aliphatic hydrocarbons) for ease and speed of application. The enamels are applied by well known techniques such as roller coating or spray 25 coating. The coatings are usually of the thermosetting type and are reacted or cured in standard 25 baking ovens.

The coatings applied to three-piece containers are not subjected to any extensive mechanical deformation in the can making process. For this reason the type of enamels for three-piece containers can be formulated from a broad range of resinous components such that when baked 30 in an oven they cure to highly cross-linked structures. This cross-linking allows a maximum 30

degree of corrosion prevention and chemical resistance to be achieved, thus affording good protection for the foodstuff or beverage ultimately packaged in the cans.

Metal containers are also made in two pieces, one being a body produced by the drawing and redrawing of flat metal blanks. For such containers, different criteria must be applied to the 35 selection of enamels. For reasons of economy, it is preferred that the metal blank be pre-coated 35 with the enamel prior to forming it into the container body by suitable dies. The forming operation places severe and extensive mechanical stresses on the enamel so its adhesion to the metal substrate and its extensibility or ability to flow without fracturing are severely tested. For this reason, formulating suitable enamels for drawn cans poses difficulties and attention must be 40 given to the blending of resins and solvents, so that on baking they form strongly adhering films 40 with the proper degree of cross-linking, to provide good corrosion and chemical resistance, while they nevertheless retain a higher level of flexibility than comparable three-piece can enamels in order to survive the forming operation without fracturing.

To control the cross-linking properly in these drawn can enamels, cross linking resins may be 45 adjusted or baking temperatures may be slightly lower than those used for three-piece can 45

enamels. So controlling cross linking tends to produce enamel coatings which are somewhat more soluble in organic solvents after baking than the enamel counterparts for three-piece cans.

This means the drawn can enamels may retain small (ppm) quantities of some solvents after the baking steps. These solvents residues can then be released after the can is packed and may 50 undesirably impair the flavor of a food product. Sometimes, however, the solvent is relatively 50 innocuous if it is present inside the packed can, but some solvents or constituents thereof can react with the food product to produce highly objectionable flavoring.

Metal containers made by multiple drawing of tin free steel can be pre-coated with various types of thermosetting organic coatings formulated with phenolic resins, epoxy resins, acrylic 55 resins, amine-type resins, polyvinyl chloride resins and co-polymers thereof, and polyester resins. 55 The "inside" and the "outside" of each sheet of plate or blank is coated; the inside is that surface of the metal which becomes the inside of the container upon drawing.

In the manufacture of 3-piece food or meat containers, sheet plate, either tin free steel or electrolytic tin plate, is coated in a separate coating operation. Since three-piece food cans may 60 require coating on both the inside and outside surfaces several passes are usually required to 60 provide adequate corrosion protection of the finished can. For example, the outside coating is put on in one pass; the inside coating may consist of one or two layers and requires a separate coating operation for the or each inside coating layer. As a result three passes through a coater can be required to manufacture 3-piece can stock. The coated plate must then be split into body 65 blanks in an operation following the coating operation. The body blanks must then be 65

2

GB2 092024A 2

transferred to a body maker where each is formed to the cylindrical shape and the longitudinal margins either soldered or welded to form the side seam. In a final operation, can bottom ends are doubleseamed to the body cylinders. These numerous operations account for considerable time and cost in the manufacture of 3-piece containers.

5 Manufacture of food containers by the newer 2-piece process of multiple drawing from precoated plate offers considerable cost savings in that a number of the previously outlined 3-piece manufacturing operations are either consolidated or eliminated. The metal stock for forming 2-piece containers in a multiple drawing process can be coil coated. In this kind of coating operation, both the inside and the outside coatings can be applied to the metal sheet in 10 a single operation thus offering manufacturing economy.

Multiple drawn cans can be made from either tin free steel or electrolytic tinplate. For economy, most containers are manufactured from tin free steel. The tin free steel can be of any number of plate thicknesses, but, the usually preferred material is 75 lb weight (34 kg) per base box, of T-4 temper continuously cast stock or 65 lb weight (29.5 kg) of DR9 temper. A base 15 box is a can maker's measure of 31,360 square inches (20.2 sq. metres) of surface (on one side) for a given weight of metal. T-4 and DR9 are steel mill designations for the hardness of the material and by inference the rolling process used to make the sheet.

Since the stock is precoated, the can forming operation subjects the coating to the same mechanical stretching and forming operations as the metal. This requires that the coating have 20 good adhesion to the metal substrate and possess the necessary mechanical properties to withstand stretching and forming without fracturing it or forceably removing it from the substrate metal. Careful selection of the organic coating materials is required, therefore. Unlike 3-piece containers, multiple drawn containers have an integral bottom and no side seam. This offers manufacturing economies since the side seam soldering or welding operation is elimi-25 nated, side seam stripe coating is eliminated and bottom end doubleseaming is eliminated. In addition, the drawn can process can be operated in a continuous manner. Thus, precoated sheet is fed to a multiple forming press while finished shells leave the press passing in a continuous manner through a flanger, beader and air tester before finally being palletized. The continuous nature of the multiple forming operation eliminates considerable labor within the manufacturing 30 process thus affording additional economies.

The severe mechanical forces imposed on the coating during multiple drawing operations would suggest rubbery or highly flexible polymeric materials should be more suitable for precoated multiple drawn containers. While rubbery materials may stretch adequately during the forming operations their chemical resistance and adhesion properties are generally inferior to 35 thermosetting coatings. Consequently, there is the paradox of flexibility necessary for multiple forming operations working against the chemical protection necessary to protect the metal from the food packed in the container. Another problem with rubbers is that flavor characteristics imparted by the vulcanizing chemicals can be absorbed by food stuffs packed in metal containers precoated with rubbers. For this reason, these materials cannot be used in metal 40 container coatings.

Coatings having good flow properties when applied to the metal surface should provide an essentially eyehole, blister free or continuous covering over the metal. This is particularly important when packing foodstuffs because any uncoated portions of the can will serve as sites for corrosion which can result in container perforations at those sites. Coating failure usually 45 occurs in the top one inch (2.5 cm) of the can nearest the end closure flange. It is well known that this is a concomitant of the metal working in terms of axial stretching and circumferential compression being at a maximum about the flange area.

A frequently used method to test the effectiveness of coverage of the coating over the metal is the enamel rater or quick test. This test is carried out with a special testing instrument and 50 essentially measures the electrical conductivity of the interior of the can when it is filled with an electrolyte.

In order to perform a quick test a specified piece of equipment is required. More particularly, a Model 1071 WACO Enamel Rater with a 0 to 1 milliamp meter attachment is used. The apparatus has an electrode which is adapted to move vertically in and out along the axis of a 55 can positioned beneath it. The electrode is positioned about 1" (2.5 cm) from the bottom of the can for the test. The can is held in position by a shuck or vice-like device which clamps it about the bottom holding it so that the open end of the can faces up toward the electrode. The can is filled with 2% solution of sodium sulfate and allowed to soak for at least 30 seconds before the electrode is lowered into the can. The solution temperature should be maintained between 72 to 60 78°F (22 to 25.5°C) and the can should be filled so that when the electrode is lowered into the test position the solution will reach approximately 1 /8" (3 mm) below the top flange radius of the can. Care should be taken to avoid wetting the flange since that will result in a false high reading. The milliamp meter of the tester is connected to the clamp device which holds the bottom of the can and to the electrode. A zeroing of the instrument is required and the operator 65 adjusts the milliamp to read "T" on the scale. Shortly after zeroing the meter a warning light

5

10

15

20

25

30

35

40

45

50

55

60

65

3

GB2 092 024A

3

comes on and the reading should be taken immediately. When this procedure is applied to properly precoated cans readings in the range of 0 to 8 milliamps should be obtained, such reading being indicative of an acceptable container.

An object of this invention has been to produce a coating system on a metal substrate which 5 will withstand the severity of multiple forming operations without destruction, so that it 5

functions efficiently to protect the metal substrate and prevent corrosion while it avoids impairing the flavor of foodstuffs. The invention has sought a coating system suitable for use with tin free steel (TFS) and tin coated steel (ETP), and which can be applied by direct roll coating or reverse roll coating.

10 According to the present invention, there is provided a precoated metal blank adapted for 10 conversion into a container body by a multiple-drawing process, wherein the surface of the blank destined to become the inside surface has a resinous coating made from a formulation comprising a phenolic resin, an epoxy resin, a polyvinyl chloride/acetate co-polymer and a polyvinyl chloride dispersion resin, the latter resin having an intrinsic viscosity of about 1.4. 15 The invention further provides a two-piece food or beverage container having a multiply-drawn 15 body produced from a metal blank coated on the surface ultimately forming the inner surface of the body with a coating system comprising a phenolic resin, an epoxy resin, a polyvinyl chloride/acetate co-polymer and a polyvinyl chloride dispersion resin dispersed in a solvent system, and the polyvinyl chloride dispersion resin having an intrinsic viscosity of about 1.4. 20 The description which follows is by way of example of the invention. 20

Because of the mechanical stretching and forming operations used in manufacturing multiple drawn containers, the coatings on the metal substrate require unique properties. A system of inside and outside coatings is required that is compatible with the multiple drawing process. An outside coating is applied by roller coating or coil coating techniques and cured in an 25 oven. For sheet coating operations, this coating is baked in a temperature range of 300 to 25

400°F (149 to 204°C) for about 6 to 10 minutes. It is usually applied to the metal substrate at a film weight of 8 to 15 mg per 4 square inches (25.8 sq. cm) of plate area. This coating can be of several chemical types, and hence can be a vinyl organosol, an epoxy resin, an amine resin, a phenolic resin or a suitably formulated blend of two or more of these resins. The outside 30 coating is generally applied at lower film weights than the inside coating for reasons of 30

economy, because it is subject to less severe corrosion attack.

The inside coating is generally applied at a film weight of 15 to 35 mg per 4 square inches (25.8 sq. cm) of plate area. The coating can be either sheet coated or coil coated. A baking temperature of 300 to 400°F (149 to 204°C) for 8 to 10 minutes is generally used in sheet 35 coating. In addition, sheet coatings can be applied in more than one pass. For example, for a 35 single drawn container, a base coating such as an epoxy type can be applied first. In a second pass, a coating of a different chemical type can be applied such as an oleoresinous enamel according to the invention. This versatility allows the can maker to apply the best inside enamel system with regard to adhesion to the metal and corrosion resistance to the packed food product 40 in question. 40

The coatings can also be applied to the metal substrate by coil coating techniques. In coil coating, both the inside and outside coating are applied at the same time. This process offers economies of manufacture because the entire coating operation is carried out in one pass.

Baking temperatures in coil coating are in the range of 500 to 600°F (260 to 315°C) for about 45 20 seconds, just enough time to remove every last trace of solvent as either the inner or outer 45 coating has about 75% solvent in its formulation.

Because of the severe mechanical deformation the coatings are subjected to during multiple drawing, the requirements for adhesion and mechanical strength are stringent. Inside coatings meeting these requirements are of the organosol type, and according to the invention consist of 50 mixtures of phenolic resin, epoxy resin, vinyl solution resins of the vinyl acetate-vinyl chloride 50 co-polymer type and high molecular weight polyvinyl chloride dispersion resins. The molecular weight range of the polyvinyl chloride has been found to be a critical factor in making successful multiple draw inside coatings. If the resin is below a certain molecular weight, the coating exhibits adhesion failure or fracturing in the top third region of the can during drawing, resulting 55 in unsatisfactory packed container performance. It has been found that a weight average 55

molecular weight greater than 110,000 is required with a preferred molecular weight greater than 150,000. The molecular weight is for instance defined by measurements made on a gel permeation chromatograph using polystyrene gel packings of known pore diameter.

The mechanical working of the precoated metal in the dies of a D. & I. press causes a rise in 60 temperature in the metal workpiece as it is formed. Temperatures in the press tooling and 60

consequently in the workpiece rise to 150°F (65°C) in the first redraw station but can go as high as 200°F (93°C) in the second redraw station, the passive heating depending on the severity of the drawing process. The beneficial effects of heating on coating performance have been shown in prior U.S. Patent No. 3,206,848. The increased temperature softens the coating 65 causing it to reflow. The range of Tg (glass transition temperature) of the coatings is important. 65

4

GB2 092024A 4

A Tg is required that is neither so high that brittle fracture occurs nor so low that softening and coating breakthrough occurs. Coating reflow is demonstrated by the increased gloss on the outside coating during drawing. This is due to the higher temperatures and pressures prevailing.

Lubrication applied to the coating is another critical factor for success in forming multiple 5 drawn containers. The lubricant provides the needed slip properties when precoated plate is formed in the press tooling. Without proper lubrication, the coatings will be scraped off by the press tools resulting in unsatisfactory containers and possible damage to the tools and dies in the press. Lubricants such as Boler wax, lanolin or petrolatrum can be used. For multiply drawn containers, petrolatum is the best with regard to tool lubrication and flavor performance. The 10 lubricant is applied by spraying from standard spray guns or fogging over the coated plate by special electrostatic waxing machines.

The integrity of the coatings is important not only as regards corrosion resistance but also as regards flavor characteristics. Residual solvents or small molecules present in the film (e.g. because of incomplete elimination during the coating bake operation) must be minimized as they 15 can impart undesirable flavors on food products. Particular care must be taken to avoid such impurities in connection with the D. & I. process which will squeeze residual solvent from the coating. Any materials and solvents left in the coating may only be ones which do not have off-flavor properties and, in addition, they must be selected to have the right amount of cross linking and must not interact with small molecules which may emanate from the food product to 20 form impurities giving undesirable flavors.

A well known property of food products such as peas, corn and beans, is the liberation of small molecules such as cystine and hydrogen sulfide after packing. These compounds arise from the breakdown of proteins in the food and can react with residual solvents and oligomers in the coating film to form reaction products which introduce undesirable flavors in the food 25 product. One source of undesirable flavors is mesityl oxide, a contaminant in certain coating solvents such as diacetone alcohol. Mesityl oxide reacts with hydrogen sulfide and can give rise to a catty odour. The reaction product, 4-methyl-4-mercoptopentan-2-one has been identified with off-flavor properties in certain canned meat and vegetable products, and has been associated with a catty off-flavor. Therefore, elimination of mesityl oxide, for example, from the 30 coating systems of drawn food cans is highly important in maintaining unimpaired flavors of foodstuffs.

This invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a partial side cross-sectional view showing a container drawn from precoated metal 35 stock according to the invention.

Figure 2 is a sectional view taken along lines 2-2 of Fig. 1,

Figure 3 is a partial side cross-sectional view similar to Fig. 1 of a three piece container side wall doubleseamed to a bottom end, the side wall being a hollow cylinder with a welded side seam, and

40 Figure 4 is a cross-sectional view taken along lines 4-4 in Fig.3.

In Figs. 1 to 4 the 2-piece container formed by multiply drawing precoated metal stock is contrasted with the 3-piece container formed by rolling and side seaming. It can be seen that fewer layers of coating are required for a 2-piece container system than a 3-piece container system. In Fig. 1 the multiply drawn can 10 has precoated inside and outside layers 11 and 12 45 (one of each) which adhere to the base metal 13 of tin-free steel or electroiytically deposited tin plate, notwithstanding the forming operation required to draw a flat precoated blank into a cup shape having a diameter usually less than its height.

In contrast, the 3-piece container 14, shown in Fig. 3, is precoated with two inside coatings 15 and 16 and one outside coating 17. This container is formed from sheet metal 18 cut into a 50 blank and rolled into a hollow cylindrical form. The cylinder is side seamed 19 along its meeting longitudinal edges by welding (as in Figs. 3 and 4), bonding or by can seaming and soldering. Notwithstanding the side seam technique selected, it is furthermore necessary to repair coat and cure the inside with a stripe 20 of side seam coating material to prevent the subsequently packed comestibles from attacking the side seam or the adjacent metal. Moreover, in fabricating 55 a three-piece container, it is required that an end 21 be doubleseamed 22 onto the hollow cylindrical container side wall, and consequently a sealing compound 23 must be used to hermetically seal that end. The additional side seam stripe 20, its curing and the doubleseaming of the bottom end 21 all contribute to the manufacturer's costs which are avoidable by the multiply drawn 2-piece container. For many applications, also, the outside of the container at 60 the side seam 24 must be protectively coated and cured. It can be appreciated that provided the proper coating systems for the inside 11 and outside 12 of a multiply drawn 2-piece containers are chosen, it becomes feasible to manufacture a low cost but safe container which will not corrode or affect the flavor of the comestibles packed therein.

The preferred outside coating 12 is primarily an epoxy resin and a slight amount lubricant in 65 combination applied as explained, with a film weight of 9 to 12 mg per 4 square inches (25.8

5

10

15

20

25

30

35

40

45

50

55

60

65

5

GB2 092 024A

5

sq. cm.). The particular epoxy coating is Mobil Chemical MC9372 and more than half the weight of material consists of epoxy resin. About 75% epoxy resin has been used successfully in the combination.

The inside coating 11 is composed of polyvinyl chloride dispersion resin, epoxy, phenolic,

5 vinyl chloride/actate copolymers as the solution resins, and preferably includes lanolin as a 5

lubricating aid during forming and titanium dioxide as a pigment. The above resins, pigment and lubricants are dispersed or dissolved in appropriate solvents to give a viscosity of 160 seconds in a number four Ford viscosity cup. Such solvents may be based on one or more aliphatic or aromatic esters or ethers. Preferably, the solvent system is 5 to 25% aromatic 10 hydrocarbon solvent(s), 10 to 15% ketonic solvent(s) and up to 60% of a solvent selected from 10 one or more ethers, esters and alcohols or mixtures thereof, the percentages being by weight of th solvent system.

Typically the solvents are slightly more than one half the total by weight; e.g. the solvents comprise 75% by weight of the composition. The composition of polyvinyl chloride, epoxy, 15 phenolic, vinyl chloride/acetate copolymers, lanolin, solvents and titanium dioxide can be 15

purchased from Midland-Dexter as MM478A. A similar formulation of the dispersion and solution resins, lubricant and pigment can be made as follows:

All Resins Except PVC 30% by weight of non-volatiles 20 PVC Dispersion Resin 43% by weight of non-volatiles 20

Pigment 27% by weight of non-volatiles

It has been found that the molecular weight of the polyvinyl chloride dispersion resin is a particularly important factor in producing a satisfactory multiple draw container precoat. If the 25 molecular weight of the polyvinyl chloride dispersion resin is below a critical value, as defined 25 by the intrinsic viscosity, the coating will lose adhesion or rupture near the top of the can because of the mechanical stretching and axial compression during severe multiple drawing operations.

Coating failure is reflected in enamel rater readings measured for the coating. Organosol 30 coatings formulated with three different molecular weight ranges of polyvinyl chloride dispersion 30 resin, sheet coated at 37 mg per 4 square inches and baked at 380°F (193°C) for 10 minutes show the following enamel rater readings after being drawn and redrawn to form a can of 3 3/16" diameter by 4 3/8" height (8.1 X 11.1 cm).

35 Enamel Rater Readings of Multiple Drawn Cans Precoated with an Inside Organosol Coating 35

Intrinsic Enamel Rater PVC Resin Viscosity Reading (m.amps)

40 A 1.02 50 40

B 1.00 45

C 1.40 2

45 As can be seen, the organosol coating with PVC resin C having the highest intrinsic viscosity 45 has the lowest enamel rater reading thus showing superior performance with respect to its formability with the metal substrate.

Flavor test packs are an important step in evaluating the performance of inside coatings in multiple drawn cans. Initial screening tests are generally carried out with three test pack media. 50 Water is used as one medium because of the high water content of many foods such as corn, 50 peas, or tomatoes. Water will quickly generate corrosion and coating adhesion will be lost if the inside coating is discontinuous, eyeholed or fractured. Chicken soup is used to test for most soups because of its fat content. These fats have a softening action on the coating. If a coating is insufficiently baked, or is not formulated properly to obtain an adequate degree of 55 cross linking, the fat in chicken soup will soften the coating causing loss of adhesion or swelling 55 and subsequent rupture of this coating film. Apple juice is used primarily as a flavor test medium and also as a material that will aggressively promote corrosion at discontinuities in the coating film. Apple juice is an important test media in characterizing the flavor properties of an inside container coating. The following table illustrates the coating adhesion performance of 60 multiple drawn cans when precoated with inside organosol coatings and test packed with those 60 three media.

6

GB2 092 024A 6

TEST PACK PERFORMANCE OF MULTIPLE DRAWN CANS PRECOATED WITH AN INSIDE ORGANOSOL COATING

5

Water Pack

Chicken Soup

Apple Juice

PVC

Intrinsic

50 min @

Pack—3

Pack—3

Resin

Viscosity

250°F (121 °C)

months months

A

1.02

Fails

Fails

Fails

10

B

1.00

Fails

Fails

Fails

C

1.40

Pass

Pass

Pass

As can be seen, the organosol coating with the highest intrinsic viscosity has the best test 15 pack performance. The foregoing data was all taken from cans coated with an organosol inside coating and baked at a temperature of 380°F (193°C) for 10 minutes. Drawn cans made from a precoated organosol coating but baked at a coil coating bake schedule of 550°F (288°C) for 20 seconds showed similar enamel rater readings and test pack results.

The glass transition temperature of the inside coating must be properly adjusted during 20 formulation so that the coating neither scuffs nor becomes brittle under the heat and pressure of the forming operations. Similarly, the composition of the resins is essential to obtaining a suitable formulation which is also resistant to chemical attack by packed products. If the glass transition is too low, the coating will soften and swell in contact with fats and it will hydrolyze with acid products. Some products may also leach out part of the coating and combine 25 therewith to give off-flavors. The glass transition temperature must be adjusted to ensure the inside or outside coatings do not degrade due to heating resulting during forming but must allow passive reflow. The glass transition temperature cannot be so high, therefore, that the coating will not exhibit some pliability and reflowability during forming. Consequently, the formulation as discussed with the proper ratios of materials, particularly solution resins, will give 30 the right amount of cross linking and therefore the appropriate glass transition temperature.

Measurements made of the apparent glass transition temperature of the inside and outside coating show the following results.

APPARENT GLASS TRANSITION TEMPERATURES OF DRAWN CAN COATINGS

Glass Transition Temperature

Inside Coating

136°F (58°C)

Outside Coating

189°F (87°C)

In forming metal containers by the draw and redraw process, precoated metal blanks are preferred to avoid more expensive post-forming coating operations. The forming operation results in considerable stretching and extension of the coating. The forming operation also 45 increases the temperature of the metal substrate and hence the coating applied to it. If the coating film contains resins below a critical molecular weight, the draw and redraw operation will open the coating film and make it less resistant to the migration of chemical species in the packed product into the film and the migration out of the film of any retained solvents, resin oligomers, etc., present in the coating film.

50 A well-known property of the food products such as peas, corn and beans is the liberation of small molecules such as cystine and hydrogen sulfide after packing. These chemical compounds arise from the breakdown of proteins in the food and they can react with the residual solvents and oligomers in the coating film to form products which introduce undesirable flavors.

It has been found that certain solvents, when added to organosol coatings can lead to the 55 formation of off-flavors in certain packed foods, especially foods known to release sulfides such as hydrogen sulfide. Examples of such foods are kidney beans or whole kernel corn. Frequently, a coating is formulated in a solvent system comprising esters and alcohols that by themselves contain no compounds which interact with sulfides. When stock solutions of the coatings are diluted in the maunfacturing plant to achieve the proper coating viscosity, certain solvents are 60 used for reasons of economy as the diluents. Exemplary diluents are (a) a mixture of xylene and diacetone alcohol and (b) xylene and isophorone. Diacetone alcohol or isophorone are condensation products of acetone that are frequently used in metal coatings as diluents because they are low cost or have excellent solvent properties for many industrial or container coatings.

While these solvents possess technically useful properties for use in coatings, both isophorone 65 and diacetone alcohol usually contain trace quantities of mesityl oxide. Mesityl oxide is highly

5

10

15

20

25

30

35

40

45

50

55

60

65

GB2 092 024A

reactive unsaturated ketonic compound which easily reacts additively with sulfides such as hydrogen sulfide to produce compounds exhibiting noxious odors which spoil the flavour of food products such as meats. When hydrogen sulfide is released in the presence of mesityl oxide, the reaction product has been identified as 4-methyl-4-mercaptopentane-2-one. The following table 5 compares flavor characteristics with residual solvent content in multiply formed cans.having a 5 height greater than the diameter.

FLAVOR TEST WITH WHOLE KERNEL CORN

10

No.

Can

Residual Diacetone Mesityl Solvent Alcohol Oxide Index (TAI) Index Index

Flavor Test Score Scale 1-9

10

15

1

Control —3 piece ETP

213 0 0

1.3

15

20

2

Multiple Drawn Organosol 1

34.0 0 0

2.6

20

25

3

Multiple

Drawn

Organosol

38.0 2.6 4.8

greater than 9

25

30

The test results show that the presence of small traces of a sulfide acceptor molecule such as mesityl oxide or its precursor, diacetone alcohol, imparts highly undesirable flavors to corn. Other food products such as kidney beans show similar trends in flavor properties.

FLAVOR TEST WITH KIDNEY BEANS

30

35

No.

Can

Coating Diluent Taste Score Solvent Mixture (Scale 1-9)

35

1

Control 3-Piece ETP

Xylene-Butyl 1.0 Cellosolve

40

2

Organosol 1

Xylene-Butyl 1.0 Cellosolve

40

45

3

Organosol 1

Xylene-diacetone 3.4 alcohol

45

The results show that the coating diluted with the xylene-diacetone alcohol mixture, in which diacetone alcohol was shown to contain mesityl oxide in the 1 -2 ppm range, produced higher taste scores than coatings diluted with xylene-butyl cellosolve.

50 Similar results have been found with organosol coating diluted with xylene-isophorone 50

mixture.

The inside coating fomulation disclosed above can be multiply formed without degradation because the resins are sufficiently pliable due to their molecular weight.

There has been disclosed herein an intermediate thermosetting coating formulation that has 55 the proper solvent release properties to allow application to the sheet metal by the lower 55

temperature sheet coating process or the higher temperature coil coating process. The precoatable resinous coating mixture provides a good corrosion barrier enamel for a metal can produced by the D. & I. method from a precoated blank, the enamel being suitable for a variety of different food products. The coating mixture withstands the mechanical stretching and 60 deformation of the multiple drawing process especially well without fracturing or loss of 60

adhesion.

The enamel solvent system contains no chemicals that confer undesirable flavor characteristics to food products and contains no solvents which can react with chemical products such as .

hydrogen sulfide given off by the food product, so the likelihood of chemical reactions leading 65 to unpalatable or noxious by-products in the food is minimal. 65

8

GB2 092024A 8

The resinous enamel and solvent mixture releases essentially all of the solvent during the baking process so that when the precoated metal is exposed to passive reheating during the multiple drawing operation residual traces of solvent or other small molecules from catalyst residues or plasticizers are not outgassed by the enamel to contribute off flavors to the packed 5 food product.

Claims (14)

1. A precoated metal blank adapted for conversion into a container body by a multiple-drawing process, wherein the surface of the blank destined to become the inside surface has a

10 resinous coating made from a formulation comprising a phenolic resin, an epoxy resin, a polyvinyl chloride/acetate co-polymer and a polyvinyl chloride dispersion resin, the latter resin having an intrinsic viscosity of about 1.4.

2. A metal blank according to claim 1, wherein the coating comprises 43% by weight of the polyvinyl chloride dispersion resin, 30% by weight of the other resins and the balance

15 comprises pigment.

3. A metal blank according to claim 1 or claim 2, wherein the polyvinyl dispersion resin has a weight average molecular weight of at least 110,000.

4. A metal blank according to claim 1, 2 or 3, wherein the coating comprises a film amounting by weight to 33 mg per 4 square inches of surface.

20

5. A metal blank according to any of claims 1 to 4, wherein its outer surface has an epoxy resin based coating thereon, this epoxy coating containing a slight amount of lubricant.

6. A metal blank according to claim 5, wherein the epoxy resin amounts to 75% by weight of the epoxy coating.

7. A metal blank according to claim 5 or claim 6, wherein the epoxy coating comprises a 25 film amounting by weight to 9 to 12 mg per 4 square inches of surface.

8. A precoated metal blank adapted for conversion into a container body by a multiple drawing process, substantially as herein described by way of example with reference to Figs. 1 and 2 of the accompanying drawings.

9. A two-piece food or beverage container having a multiply-drawn body produced from a 30 metal blank coated on the surface ultimately forming the inner surface of the body with a coating system comprising a phenolic resin, an epoxy resin, a polyvinyl chloride/acetate copolymer and a polyvinyl chloride dispersion resin dispersed in a solvent system, and the polyvinyl chloride dispersion resin having an intrinsic viscosity of about 1.4.

10. A container according to claim 9, wherein the polyvinyl chloride dispersion resin is 43% 35 by weight of all the non-volatiles in the system, all other resins are 30% by weight of all the said non-volatiles and the balance of the said non-volatiles comprises pigment.

11. A container according to claim 10, wherein the pigment is titanium dioxide.

12. A container according to claim 9, 10 or 11, wherein the glass transition temperature resulting from the combination of resins is such as permits passive reflow of a coating film on

40 metal as a consequence of temperatures reached during forming by the container drawing operation.

13. A container according to claim 9 and substantially as herein described by way of example with reference to the accompanying drawings.

14. Any novel precoating enamel system disclosed herein and formulated from a phenolic 45 resin, an epoxy resin, a polyvinyl chloride/acetate co-polymer and a polyvinyl chloride dispersion resin dispersed in a solvent system, the said dispersion resin having an intrinsic viscosity of about 1.4.

5

10

15

20

25

30

35

40

45

Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd.—1982.

Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.